Glycerol is a sweet, highly viscous fluid that’s very good at absorbing moisture from the ambient air. That’s why a drop of pure glycerol in laboratory conditions quickly develops convection cells – even when upside-down, as shown above. This is not the picture of Bénard-Marangoni convection we’re used to. There’s no temperature or density change involved; in fact, there’s no buoyancy involved at all! This convection is driven entirely by surface tension. As glycerol at the surface absorbs moisture, its surface tension decreases. This generates flow from the center of a cell toward its exterior, where the surface tension is higher. Conservation of mass, also known as continuity, requires that fresh, undiluted glycerol get pulled up in the wake of this flow. It, too, absorbs moisture and the process continues. (Image credit: S. Shin et al., pdf)
Year: 2017
Bubbles Sliding
Two-phase flows involve more than one state of matter – in this case, both gas and liquid phases. Flows like this are common, especially in applications involving heat transfer. In some heat exchangers, bubbles will rise and then slide along an inclined surface, as shown above. The motion of these bubbles is a complicated interplay between the surface, the bubble, and the surrounding fluid. The bubble’s wake, visualized here using schlieren imaging, is unsteady and turbulent. Although the bubble oscillates in its path, the wake spreads even wider, and its turbulence stirs up the liquid nearby, increasing the heat transfer. (Image and research credit: R. O’Reilly Meehan et al., source)
Build Your Own Fluidized Bed
Previously, we featured some GIFs of bubbling, fluidized sand (below). Inspired by the same video, Dianna from Physics Girl decided to build her own set-up, discovering along the way that it’s a little tougher than you might think. To work well, you’ll need very fine, dry particles and a good way to uniformly distribute the air so it doesn’t simply bubble up in one spot. And if you accidentally apply too much air pressure, you may get a face full of sand. The final results are very fun, though, and hopefully Dianna’s lessons learned will help any other DIYers interested in trying this experiment at home. For a little more on the physics here and in related topics, check out some of our previous posts on fluidization, soil liquefaction, quicksand, and dam failures. (Video credit: Physics Girl; image credit: R. Cheng, source)
Bioluminescent Plankton
In nutrient-rich marine waters, dinoflagellates, a type of plankton, can flourish. At night, these tiny organisms are responsible for incredible blue light displays in the water. The dinoflagellates produce two chemicals – luciferase and luciferin – that, when combined, produce a distinctive blue glow. The plankton use this as a defense against predators, creating a flash of blue light when triggered by the shear stress of something swimming nearby. The dinoflagellates respond to any sudden application of shear stress this way, so they glow not only for predators, but for any disturbance – mobula rays (above), sea lions, boats, or even just a hand splashing in the water. In person, the experience feels downright magical. I had the opportunity to experience bioluminescence in the Galapagos last year. The light from the dinoflagellates is incredibly difficult to film because it can be so dim, but as the BBC demonstrates, it’s well worth the effort it takes to capture. (Image credit: BBC from Blue Planet II and Attenborough’s Life That Glows; video credit: BBC Earth)
Cavitating Inside a Tube
Cavitation – the formation and collapse of low-pressure bubbles in a liquid – can be highly destructive, shattering containers, stunning prey, and damaging machinery. Inside an enclosure, cavitation can happen repeatedly. Above, a spark is used to generate an initial cavitation bubble, which expands on the right side of the screen. After its maximum expansion, the bubble collapses, forming jets on either end that collide as the bubble shrinks. Shock waves form during the collapse, too, although in this case, they are not visible.
Those shock waves travel to either end of the tube, where they reflect. The reflected waves behave differently; they are now expansion waves rather than shock waves. Their passage causes lower pressure. The two expansion waves meet one another toward the left end of the tube, in the area where a cloud of secondary cavitation bubbles form after the first bubble collapses. Pressure waves continue to reflect back and forth in the tube, causing the leftover clouds of tiny bubbles to expand and contract. (Image credit: C. Ji et al., source)
Peering Between Particles
Turbulence is not the only way to mix fluids. Even a steady, laminar flow can be an effective mixer if geometry lends a hand. Above, two dyes, fluorescein (green) and rhodamine (red), are injected into a porous flow through packed spheres. The flow runs from bottom to top in both images. Seeing the flow in such a crowded geometry is challenging. Here researchers used spheres with an index of refraction that matches water – that helps them avoid refraction that would prevent them from looking through spheres to the flow on the other side. They also lit a narrow plane of the flow using a laser sheet to isolate it. Together, this allowed the researchers to track the mixing of the two initially separate streaks of dye as they randomly mix in the spaces between spheres. (Image and research credit: M. Kree and E. Villermaux)
“Macrocosm”
In “Macrocosm” artist Susi Sie explores a liquid world of black and white. The two colors diffuse and mix to a soundtrack of “space sounds” recorded by NASA. (Most of these are probably ionic sound rather than sound as we’re used to, but even that is somewhat fluid dynamical.) The result is beautiful, surreal, and more than a little creepy. Happy Halloween! (Video and image credit: S. Sie)
The Cheerios Effect
You’ve probably noticed that cereal clumps together in your breakfast bowl, but you may not have given much thought as to why. This tendency for objects at an interface to attract is known as the Cheerios effect, although it happens in more than just cereal, as Joe Hanson from It’s Okay to Be Smart explains. The effect is a combination of buoyancy, gravity, and surface tension acting in concert.
When air, a liquid, and a solid meet, they form a meniscus, the curvature of which depends on characteristics of their interaction. Light, buoyant cereal and the walls of your bowl both have upward-curving menisci. Denser objects, like the tacks shown below, stay at the surface only because surface tension holds them up. Their meniscus curves downward.
Objects with a similar meniscus curvature will attract. For cereal approaching a wall, the light Cheerio is buoyant enough that there’s an upward force on it, but it’s constrained to stay at the interface. It cannot rise, but that buoyancy is enough to let it climb the meniscus at the wall. The two tacks attract one another for similar reasons, except this time their weight helps them fall into one another. Check out the full video to see more examples of this effect in nature! (Video and image credit: It’s Okay to Be Smart; research credit: D. Vella and L. Mahadevan, pdf)
Forming Craters
Asteroid impacts are a major force in shaping planetary bodies over the course of their geological history. As such, they receive a great deal of attention and study, often in the form of simulations like the one above. This simulation shows an impact in the Orientale basin of the moon, and if it looks somewhat fluid-like, there’s good reason for that. Impacts like these carry enormous energy, about 97% of which is dissipated as heat. That means temperatures in impact zones can reach 2000 degrees Celsius. The rest of the energy goes into deforming the impacted material. In simulations, those materials – be they rock or exotic ices – are usually modeled as Bingham fluids, a type of non-Newtonian fluid that only deforms after a certain amount of force is applied. An everyday example of such a fluid is toothpaste, which won’t extrude from its tube until you squeeze it.
The fluid dynamical similarities run more than skin-deep, though. For decades, researchers looked for ways to connect asteroid impacts with smaller scale ones, like solid impacts on granular materials or liquid-on-liquid impacts. Recently, though, a group found that liquid-on-granular impacts scale exactly the way that asteroid impacts do. Even the morphology of the craters mirror one another. The reason this works has to do with that energy dissipation mentioned above. As with asteroid impacts, most of the energy from a liquid drop impacting a granular material goes into something other than deforming the crater region. Instead of heat, the mechanism for dissipation here is the drop’s deformation. The results, however, are strikingly alike.
For more on how asteroid impacts affect the moon and other bodies, check out Emily Lakdawalla’s write-up, which also includes lots of amazing sketches by James Tuttle Keane, who illustrates the talks he hears at conferences! (Image credits: J. Keane and B. Johnson; via the Planetary Society; additional research and video credit: R. Zhao et al., source; submitted by jpshoer)
Lenticular Clouds
Lenticular clouds are peculiar enough that, for years, they’ve been mistaken for other things – often UFOs. These lens-shaped clouds tend to form near mountainous terrain, where air gets forced up and over the topology. If there’s a drop in temperature as the air rises, water can condense out to form the cloud. Once the air sinks, it warms enough that condensation is no longer possible. The result is a cloud that appears to stand still even though the air is moving. In reality, the cloud is constantly reforming from the moisture of incoming air. Lenticular clouds can form as a single layer, or they can form stacks like the one pictured above in Boulder, Colorado. They may seem odd, but they’re actually fairly common. If you live near hills or mountains, keep an eye out for them! (Image credit: @bayouowl; via Ilya L.)
